rman backup to tape

I am trying to develop a strategy for RMAN and have the following scenario:

I have a disk for dedicated backup disk and a backup tape drive.

I would like to first backup the database to disk for reasons of speed and availability and then use RMAN/ LSM to copy those backups the existing backups on disk to tape for extra security in case the backup drive should fail.

Questions:

How do i copy the backups to disk-
How do i know what files to copy? and
Can i use a datafilecopy with the channel defined as   type 'sbt_tape'?

Has anyone got similar setup and can help.

thanks a lot,
Paul

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